With grooved rocker tips, it is near impossible to set the valve lash to specs with a feeler gauge, since the feeler gauge will be bridging across those divots in the tips, giving you a larger gap than what is actually measured.
I haven't tried this yet, but it would seem possible to set grooved rockers with thinner feeler gauges to account for the depth of the divots. You would have to use trial and error to find the correct gauge thickness and hope that all of them are worn evenly (they seem to go that way generally).
I would only try this if I was trying to put off tearing the rocker assembly apart until I was intending to do work to the head in the near future.
